The end wall is the part of thermoplastic containers that is the most difficult to treat and that requires the most attention during the manufacturing process.
The end wall is also the thickest part of the containers, at least in parts: typically the thickness of the end wall can be between one and twenty times the thickness of the wall of the container body at some points at least. As a result, the end wall accumulates a large amount of heat during the heating step performed prior to the molding step. This heat is still present when the finished container is removed from the mold.
The hot end wall is mechanically very fragile, because the still very hot thermoplastic material can become deformed from its own weight and/or from the residual stresses associated with the blowing process. Having a perfectly shaped container end wall ensures container stability when it is placed on a flat surface, and any deformation of the end wall compromises this stability and results in a commercially unusable container.
It is therefore of great importance that the material that forms the end wall be cooled as quickly as possible when the finished container leaves the mold, so that it is stabilized in the required shape imparted by the mold. In any event it is desirable for the shape of the end wall to be stable, and therefore for the temperature be lowered, when the container reaches the next processing station.
In the past, the end walls of containers leaving a molding unit could be cooled naturally by the ambient atmosphere as they traveled the path leading to the next station (for example a filling machine), when this path was sufficiently long (e.g. a few dozen seconds).
Some systems have also sprayed a mist of gaseous fluid (usually air) or a liquid (usually water) in the form of a possibly cooled mist, onto the end walls of the containers filing out of the molding system, in order to accelerate the cooling of the end walls. However, this type of arrangement complicates the structural design of already highly encumbered systems and also increases manufacturing and maintenance costs. Furthermore, coolant sprays can cause pollution (appearance of bacteria, etc.), both from the blowing system and the final containers, as well as disturbances in adjacent parts of the system. For this reason, avoiding such arrangements is preferred.
Finally and most importantly, the operators of thermoplastic container production facilities require ever-increasing operating speeds, and rates on the order of 80,000 containers per hour are currently anticipated. In addition, the need for increasingly compact systems in order to fit into smaller spaces is leading to closer proximity of the various processing stations, for example between the blow-molding system and the filling system that follows it. This results in a considerable shortening of the container transit time. For example, the time for transferring containers between the blow-molding system and the filling system may be reduced to only a few seconds. It is no longer possible under such conditions for the end walls of containers leaving the molding unit to be cooled under the conditions which have applied in the past.
Independently of what has just been described concerning the cooling of hot end walls of thermoplastic containers leaving the molding unit, and in the general context of thermoplastic container manufacturing, some production processes may require the application of a product in a fluid form, particularly a liquid, to the end wall of thermoplastic containers, not to lower the temperature as described above, but rather to raise the temperature or to modify the surface finish of the container end wall. In at least some cases, the surface finish of the container end wall may be modified by applying an appropriate fluid, particularly a liquid, to the outer face of said end wall. Examples include depositing a layer to protect against corrosive chemicals or coloring the outer face of the end walls for decorative purposes.
A system for manufacturing containers by molding is known from document GB 1 375 041. The system comprises three primary stations. At the first station, the containers are molded in cavities arranged in a rotating wheel. Outside this wheel and near its peripheral edge is a means for cooling the containers in the cavities. This cooling means comprises a rotating roller, to which is attached a spongy mass which dips into a container of cooling liquid.
Such a cooling means may result in container deformations.